The present invention relates to polytetrafluoroethylene resin tubes having small pore size simultaneously with a high porosity and to a process for producing the same.
Although nonporous tubes or pipes composed of polytetrafluoroethylene resin having various inside diameters and wall thicknesses have been on the market, only quite limited kinds of porous tubes or pipes have been produced. Processes for producing porous polytetrafluoroethylene resin tubes are described in Japanese Patent Publications Nos. 13560/67 and 18991/76. The process for producing porous polytetrafluoroethylene resin tubes as described in these patent publications comprises molding a polytetrafluoroethylene resin by a paste method to form a tube and heating it to 327.degree. C. or more in stretched state. The tubes produced by such processes have a microstructure comprising nodules linked to one another by fine filaments, and the space surrounded by the filaments and the nodules defines a pore. As a result of various studies about tube molding, the present inventors have found that prior art moldings only having comparatively large pore size as large as 1.mu. or more are generally obtained by these known processes and they have a defect that porosity remarkably decreases when the pore size is 0.5.mu. or less.
On the one hand, in spite of being nonporous or porous, there is a definite relation between the inside diameter and the thickness of the polytetrafluoroethylene resin tubes. Namely, if the inside diameter increases, the thickness has a tendency of increase and the ratio of the thickness to the inside diameter is generally larger than 0.1. This depends upon the method of producing the polytetrafluoroethylene resin, such as the paste extruding method, etc. The reasons for this are that the tube before sintering at about 327.degree. C. or more is very brittle and the tube is more easily broken by slight exterior force if the inside diameter becomes larger. For use in filtration or separation of a mixture such as solids, liquids and gases, etc., it is often preferred to have a small thickness irrespective of the inside diameter of the tube.